To the non-botanist the algae are a mystery. He may be aware that the seaweeds and the pond "blooms" are algae, but is unlikely to be aware of the almost universal distribution of these complex organisms in moist environments. The direct economic importance of the algae is considerable and is likely to increase rapidly as the human population of the world expands faster than the conventional sources of protein foodstuffs. Some species,too, produce toxic secretions which are dangerous to domestic animals and to man himself.

It is certain that man's contact with algae will increase, and it is probable that the role of algae in human disease either as opportunistic pathogens (e.g. Protothecosis, Davies 1972) or as cutaneous irritants or sensitisers has been underestimated, because unsuspected.

Even to the botanist the algae present considerable problems. They are difficult to classify, and even define. The outline which follows is based on the authoritative works by Prescott (1969) and by Chapman & Chapman (1973). Algae may be loosely defined as chlorophyll-bearing organisms (and their colourless relatives) which are thalloid - that is to say, have no true roots, stems or leaves. Some algae are commensally associated with fungi to form lichens.

Botanical classifications are based on morphology and are an indication of presumed evolutionary relationships. As has been shown in the account of many families of flowering plants, botanically closely related genera are more likely to show chemical similarities than are those less closely related, although there are notable exceptions to this generalisation. There are a number of controversial features in any available classification of the algae, but the taxonomists agree about the grouping of algae into classes. though they differ over the grouping of the classes into phyla or divisions. Following Chapman & Chapman (1973) the following phyla or divisions may be listed. The genera so far proved to be of interest to the dermatologist are the only ones included.

Phycocyanin in blue-green algae has been suspected of allergenicity on the basis of patch tests (Cohen & Reif 1953). A toxic principle which may play some part in causing dermatitis has been isolated (Chu 1969, Moikeha & Chu 1971, Moikeha et al. 1971).

A hepatotoxic and photosensitising substance is present in Microcystis sp. and other water-blooms (Kingsbury 1964).

Dermatitis

Four clinical syndromes have been attributed to the effects of algae. An erythematous papulovesicular eruption on areas of skin not protected by a bathing costume was reported in a child who had swum in a lake with a "bloom" of blue-green algae. Patch tests to Anabaena were positive. Another blue-green algae, Lyngbya majuscula causes seabather's dermatitis in Hawaii (Grauer & Arnold 1961). The dermatitis may effect only skin covered by a tight fitting costume (Grauer 1959). Blue-green algae of various genera produced toxic or allergic dermatitis in Czechoslovakia (Krauskopf 1973). Other cases in bathers were reviewed by Schwimmer & Schwimmer (1955) and Hardin (1961).

An eczematous eruption beginning on the hands, and sometimes becoming more extensive, has been shown (Fraser & Lyell 1963, Beer et al. 1968, Beer & Jones 1969) to be caused by the green algae Fragillaria striatulata, which becomes established in the summer months on the ropes of lobster pots.

"Redfeed dermatitis" may ultimately be caused by certain coloured algae, perhaps Gymnodinium, which support the orange-red crustaceans, known as "redfeed". Fish which have eaten this crustacean in any quantity do not keep well after being caught and fishermen and processors who handle them may develop irritation and ulceration of the skin of the hands. "Redfeed" dermatitis has been reported from the Baltic (Kersten & Zschunke 1955, Bowszyc 1966) and from the United States (Schwartz & Tabershaw 1945). The role of algal toxins is unproven.

An atopic sensitivity to algae has been reported. Heise (1949) found blue-green algae responsible for seasonal rhinitis, related to swimming in lakes. Many blue-green and green algae are airborne in significant numbers at certain seasons. This observation led McElhenney et al. (1962) to skin test with algae children with respiratory allergic disorders; they obtained some positive reactions. Six strains of green algae commonly encountered in air, water and soil samples in the United States were used to skin-test 79 atopic patients; 47 gave positive results; species of Chlorella, Chlorococcum and Scenedesmus were chiefly involved (Bernstein & Safferman 1966). Further investigations (Bernstein & Safferman 1973) showed a high incidence of positive skin tests to a Chlorella species found in house dust, in patients with perennial rhinitis. Urticaria may accompany the rhinitis.

Sensitisation to the algal component of lichens has also been reported.

Photosensitisation has been reported in livestock drinking water containing heavy concentrations of blue-green algae (Wild 1961, Kingsbury 1964, Schwimmer & Schwimmer 1968). It accompanies blood in the faeces and icterus in those few cases of poisoning that are not rapidly fatal.

Subcutaneous injection into rats of carragenin derived from these species induces sensitivity to cold. Necrotising lesions occur in the nose, paws and tail (Selye 1965). Carragenin is said to stimulate collagen production in wounds (Grant 1962). Injection of carragenin into the cornea of guinea pigs causes granuloma formation (Burns & Beighle 1960).